Connecticut is divided into four main geographic regions. The Connecticut and Quinnipiac river valleys form the Central Lowlands, which bisect the state in a north–south direction. The Eastern Highlands range from 500 ft (150 m) to 1,100 ft (335 m) near the Massachusetts border and from 200 ft (60 m) to 500 ft (150 m) in the southeast. Elevations in the Western Highlands, an extension of the Green Mountains, range from 200 ft (60 m) in the south to more than 2,000 ft (600 m) in the northwest; within this region, near the Massachusetts border, stands Mt. Frissell, the highest point in the state at 2,380 ft (726 m). The Coastal Lowlands, about 100 mi (160 km) long and generally 2–3 mi (3–5 km) wide, consist of rocky peninsulas, shallow bays, sand and gravel beaches, salt meadows, and good harbors at Bridgeport, New Haven, New London, Mystic, and Stonington.

Connecticut has more than 6,000 lakes and ponds. The two largest bodies of water—both artificial—are Lake Candlewood, covering about 5,000 acres (2,000 ha), and Barkhamsted Reservoir, a major source of water for the Hartford area. The main river is the Connecticut, New England's longest river at 407 mi (655 km), of which 69 mi (111 km) lie within Connecticut; this waterway, which is navigable as far north as Hartford by means of a 15-ft (5-m) channel, divides the state roughly in half before emptying into Long Island Sound. Other principal rivers include the Thames, Housatonic, and Naugatuck.

Connecticut's bedrock geology and topography are the product of a number of forces: uplift and depression, erosion and deposit, faulting and buckling, lava flows, and glaciation. About 180 million years ago, the lowlands along the eastern border sank more than 10,000 ft (3,000 m); the resultant trough or fault extends from northern Massachusetts to New Haven Harbor and varies in width from about 20 mi (32 km) to approximately 4 mi (6 km). During the Ice Ages, the melting Wisconsin glacier created lakes, waterfalls, and sand plains, leaving thin glaciated topsoil and land strewn with rocks and boulders.